Conventional methods for flame retardation of flammable resins include those which comprise adding chloride-containing compound, bromide-containing compound or antimony trioxide to resins; and incorporating chloride-containing compound or bromide-containing compound into resins. However, it has been said that these methods are not preferred from the viewpoint of environmental protection and toxicity, and improvement of flame retarding methods has been required. As to the flame retarding methods devoid of chloride-containing compound or bromide-containing compound, the method using phosphorus-based flame retardants have been investigated.
It has been said that the mechanism of flame retardation for resins is to block heat and oxygen supply to a burning resin with a film of polyphosphate phase formed on the resin surface during burning and a char layer formed by dehydration of the resin. This mechanism is considered to be particularly effective against resins for which it is easy to form a char film, that is, easy to dehydrate. While, for the flame retardation of a resin for which it is hard to form a char film by dehydration with phosphorus or a phosphor compound, an amount of phosphorus or a phosphor compound to be incorporated into the resin needs to be increased since the flame retardation of the resin mainly depends on a film formed by polyphosphate phase.
For flame retardation of resins for which it is hard to form a char film without increasing the amount of phosphorus and phosphor compounds, it will be conceived to employ a flame retardant resin composition incorporating a component to be a raw material of a char film in advance.
Patent Document 3 proposes a method using a crosslinked phosphazene compound and a polyphenylene ether resin as a flame retardant for flame retardation of polyalkylene allylate-based resin. The crosslinked phosphazene compound and polyphenylene ether resin impart good flame retardancy to a polyalkylene allylate-based resin. However, the polyalkylene allylate-based resin containing the crosslinked phosphazene compound and polyphenylene ether resin as a flame retardant is not sufficient enough in processability, thermal resistance, mechanical property, dielectric characteristic, and appearance of the molded article thereof.
There has also been disclosed an art for flame retardation by adding a phenol resin (to the flammable resin) having a weight average molecular weight in polystyrene equivalent of 500,000 or more (Patent Document 4). However, the addition of such a high molecular weight phenol resin unfavorably causes considerable deterioration in processing flowability and effect of imparting flame retardancy.
Further, there has been disclosed an art achieving flame retardation without deteriorating of inherent mechanical properties of a resin by using a novolac-type phenolic resin having a weight average molecular weight in polystyrene equivalent of 5,000 or more and less than 50,000 and containing unreacted phenol in an amount of less than 0.5 wt % of (Patent Document 5). However, this art does not teach any technical thought that a ratio of high molecular weight component and low molecular weight component contained in the phenol resin in a specific range suppresses lowering of thermal resistance while maintaining excellent flame retardancy, mechanical properties and processability, that is, it does not suggest the present invention.
Patent Document 6 discloses a method for flame retardation of polyalkylene terephthalate resin by using a phenol-based resin and a phenoxyphosphazene in combination. According to this method, however, it is not possible to impart to a resin flame retardancy, thermal resistance, impact strength, moldability (appearance) and mechanical strength in good balance since a full investigation about phenol-based resins has not been made.
These references do not lead to the effects attained by the present inventors, namely, the effects that a flame retardant resin comprising specific phenol resins and phosphate compounds maintain or improve not only flame retardancy but also various properties such as thermal resistance, mechanical property, workability, and low smoke evolution in good balance. It was the present invention to achieve the excellent effects as described hereinafter.
Patent Document 1: Japanese Patent Publication No. 3-73590
Patent Document 2: Japanese Patent Application Laid-Open No. 8-225714
Patent Document 3: International Publication No. WO 03/002666
Patent Document 4: Japanese Patent Application Laid-Open No. 2000-273132
Patent Document 5: Japanese Patent Application Laid-Open No. 2001-164256
Patent Document 6: International Publication No. WO 01/048086